A significant portion of the domestic foamed polystyrene industry is threatened by environmental problems associated with production and use of expanded, or foamed polystyrene. Foamed (also known as "expanded" or "cellular") polystyrene is produced via physical blending of blowing agents with the polymer matrix, followed by heat treatment to initiate foaming. Blowing agents can be sub-divided into two general classes: (1) small molecules which thermally decompose to a gas (predominantly nitrogen) which initiates foaming, plus other volatile fragments; and (2) volatile liquids which are absorbed into the polymer, and which subsequently foam the polymer upon heating simply via vaporization. Examples of the first class of blowing agents include azo compounds (nitrogen producing) and calcium carbonate (CO.sub.2 -producing). Examples of the second type of blowing agent include Freons, pentane, air, nitrogen, and carbon dioxide.
The cells formed in the polymer during foaming remain filled with vapor characteristic of the blowing agent used. Given the large volume of foam produced each year, diffusion of vapor into the atmosphere during the foamed product's lifetime can cause significant environmental problems. Further, refoaming of recycled foamed polystyrene entails production and use of additional blowing agent which is both expensive and increasingly environmentally unacceptable.
Conventional foamed thermoplastics are produced via two distinct processes. In the first process, blowing agent is added either just prior to, or during extrusion of the polymer. High pressure within the extruder maintains a homogeneous mixture of blowing agent gas and polymer. Foaming commences with the reduction of pressure upon the polymer exiting the extruder. In the second process, blowing agent is added to polymer beads, which are then stored and shipped to a molder. Upon heating in a mold, the beads expand because of the action of the blowing agent, producing a molded, foamed article such as the drinking cups and trays used in many food-service applications.
Until recently, the preferred blowing agent for use in polystyrene beads was a Freon (chlorofluorocarbon) material or mixture. Freons were preferred because of their low toxicities, low flammabilities, and low boiling points. However, the publicity surrounding the effect of Freons on the atmospheric ozone layer, followed swiftly by the Montreal protocols, prompted the plastic industry to replace Freons in expanded polystyrene by a volatile alkane, usually pentane. Because pentane is not a "natural" material (it is refined from petroleum) and is also a "greenhouse gas", concern has arisen over the climactic effects of significant amounts of pentane released into the atmosphere as a result of foamed polystyrene production. The plastics industry has thus been searching for environmentally acceptable replacements for pentane in foamed polystyrene.
From an environmental perspective, CO.sub.2 is unquestionably an attractive thermoplastic blowing agent in that it can be readily recovered from the atmosphere, it is non-flammable, and it exhibits relatively low toxicity. Indeed, experience has shown that carbon dioxide can produce a cellular morphology when used as a blowing agent for polystyrene. U.S. Pat. No. 4,925,606, German Patent No. 3,829,630, U.S. Pat. No. 4,911,869, Japanese Patent No. 63,000,330, Zwolinski, L. M., Dwyer, F. J., 42 Plast. Eng. 45 (1986) and French Patent No. 2,563,836 discuss the use of carbon dioxide as a blowing agent in extruded polystyrene foam.
As indicated by Wissinger, R. G. and Paulaitis, M. E., 25 J. Polym. Sci.: Part B: Polym. Phys. 2497 (1987) polystyrene will indeed absorb significant amounts of CO.sub.2 under pressure. Because of the relatively low solubility of CO.sub.2 in polystyrene and its high volatility, however, most of the gas rapidly effuses from the polymer matrix upon reduction of the pressure. This rapid effusion prevents formation of a commercially acceptable foam. Commercial producers of extruded polystyrene foam, therefore, use either Freon, pentane or a mixture of Freon or pentane and CO.sub.2 as a blowing agent.
Moreover, despite the limited success of CO.sub.2 in extruded foam, either alone or in a mixture, the use of CO.sub.2 in foamed polystyrene beads is not presently possible. Quite clearly, unlike pentane and freon, the equilibrium concentration of CO.sub.2 in polystyrene at atmospheric pressure is too low to support subsequent foaming of polystyrene beads or secondary foaming of slabstock during thermoforming.
In an art unrelated to foamable polymers, it is known that low molecular weight primary and secondary amines will react with CO.sub.2 to form carbamic "zwitterions," providing the amines are sufficiently basic in character. These reactions have been discussed by Javier, F. J. B. G., Ing. Quim. 317 (October 1989); Javier, F. J. B. G., Ing. Quim., 215 (November 1989); Danckwerts, P. V., Sharms, M. M., 10 Chem. Eng. 244 (1966); Laddha, S. S., Dankwerts, P. V. 37 Chem. Eng. Sci. 475 (1982); Versteeg, G. F., an Swaaij, W. P. M., 43 Chem. Eng. Sci. 573 (1988) and Danckwerts, P. V., 34 Chem Eng. Sci 443 (1979). It is also known that these zwitterions are stable at ambient conditions but will revert to carbon dioxide and free amine at higher temperatures. U.S. Pat. No. 3,029,227; U.S. Pat. No. 3,423,345; U.S. Pat. No. 4,102,801. CO.sub.2 /amine reactions have previously been used to construct thermally reversible protecting groups for reactive epoxy systems and to selectively remove carbon dioxide and acid gases from gas streams.
General public awareness concerning the protection of the environment has created the need to devise environmentally friendly and energy-efficient technology for the clean-up of industrial gas streams. Weakly acidic gases such as CO.sub.2, SO.sub.2, NO.sub.x and H.sub.2 S discharged directly into the atmosphere have been suggested to contribute to the formation of acid rain and so-called greenhouse warming. In addition, acidic gases such as CO.sub.2, SO.sub.2 etc. can act as poisons for various catalyst systems and thus must be removed from certain process streams. Traditional methods for acid gas removal include:
1. Aqueous solutions of amines/alkanolamines for scrubbing CO.sub.2 as discussed above. The main disadvantages of these sorbent systems are: Slow reaction rates, energy intensive regeneration step (must heat large volumes of water), side reactions which degrade the amines, and loss of amines by evaporation.
2. Limestone for the removal of SO.sub.2 ; the major environmental drawback associated with this process is the generation of large quantities of sludge.
3. High temperature mineral sorbents for the removal of SO.sub.2 and NO. These materials are however not designed to also remove CO.sub.2 selectively.
It is an object of this invention to provide environmentally safe, foamed polymers and especially to provide a polymeric matrix incorporating pendant amine groups capable of reversibly complexing CO.sub.2, thereby providing an environmentally safe method of producing an expanded or foamed polymer.
It is also an object of this invention to provide polymeric matrices incorporating amine groups useful generally as thermally reversible sorbents for acid gases.